562 GENERAL PHYSIOLOGY 



no oxygen. In this condition no pseudopodia were formed for days. 

 When, however, he let a few small bubbles of air into the vessel, 

 the extension of pseudopodia immediately began, and after five 

 hours the lump of protoplasm had extended upon the inner wall of 

 the vessel into a richly-branched network. It is evident from this 

 that it is the chemical affinity of certain portions of the 

 protoplasm for oxygen that diminishes the surface-tension at 

 definite places, and so leads to the formation of pseudopodia. With 

 unilateral action of oxygen this must lead to positive chemotaxis, 

 as has actually been demonstrated by Stahl ('84) in naked proto- 

 plasmic masses. As regards the manner in which the chemical 

 affinity of the protoplasmic particles for the oxygen of the medium 

 diminishes the surface-tension of the drop, it may at least be 

 imagined that by the introduction of the oxygen-atoms into the 

 biogen-molecules the cohesion of the latter is diminished. 



In non-living nature there is a very striking analogue of the 

 fact that amoeboid changes of form and movements are caused in a 

 drop by the chemical affinity of certain constituents of the drop for 

 substances in the surrounding medium. This is afforded by the 

 interesting experiments of Gad ("78) upon the behaviour of oil- 

 drops in alkaline media, which later were studied by Quincke ('88). 

 It is well known that rancid fats and oils contain molecules of free 

 fatty, or oily, acids between the molecules of pure fat or oil. Upon 

 contact with alkalies these acids combine with them to form 

 soluble soaps. Hence, if a drop of rancid oil be put into a feebly 

 alkaline liquid, a continual formation of soap takes place at the 

 surface of contact of the two. Thereby the surface-tension is 

 diminished locally here and there, and there results a genuine 

 formation of pseudopodia by the oil-drop. By varying the alkalinity 

 of the medium and the amount of free acids in the oil-drop, a great 

 variety in the forms of the processes can be produced, many of the 

 latter presenting a startling similarity to the forms of pseudopodia 

 jrf certain Rhizopoda (Fig. 269). 



If, thus, by the chemical affinity of certain particles of a drop 

 for substances in the surrounding medium the surface-tension is 

 diminished, vice versa an increase of surface-tension must come 

 about by increased attraction between the particles of the drop. 

 Such an increase of cohesion between the biogen-molecules 

 themselves, or between them and other constituents of the cell- 

 body, is comprehensible when it is borne in mind that the extent 

 of the molecular attraction is influenced by changes in the chemical 

 constitution of the molecules. It has been seen above that the 

 cohesion is diminished by the oxidation of the latter. If now they 

 be decomposed, the idea is strongly suggested that this profound 

 change in their chemical constitution is associated with an increase 

 of cohesion. 



Upon the basis of this idea, the following picture may be drawn 





